Control circuit of interleaved switching power supply

ABSTRACT

In one embodiment, a control circuit configured for an interleaved switching power supply having first and second voltage conversion circuits, can include: a feedback compensation signal generation circuit that generates a feedback compensation signal; a first power switch control circuit that activates a first on signal when a first voltage signal that represents an inductor current of the first voltage conversion circuit is less than the feedback compensation signal, a first power switch of the first voltage conversion circuit being turned on based on the first on signal, and turned off after a predetermined time; and a second power switch control circuit that activates a second on signal after half of a switching period from a rising edge of the first on signal, and a second power switch control signal to turn on a second power switch of the second voltage conversion circuit based on the second on signal.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 201310432481.9, filed on Sep. 18, 2013, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a switch mode power supply, and more particularly to control circuitry of an interleaved switching power supply.

BACKGROUND

Switch mode power supplies can efficiently convert electrical power from a source to a load, or to several different loads, with each corresponding to a different output. The main transistor of a switching-mode supply can switch between on and off states at a given operating frequency, and voltage regulation can be achieved by varying the ratio of the on-to-off time of the main transistor. Switch mode power supplies may have relatively high power conversion efficiency, as compared to other types of power converters. Switch mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight.

SUMMARY

In one embodiment, a control circuit configured for an interleaved switching power supply having first and second voltage conversion circuits, can include: (i) a feedback compensation signal generation circuit configured to calculate and compensate an output voltage sampled at output terminals of the interleaved switching power supply, and to generate a feedback compensation signal; (ii) a first power switch control circuit configured to activate a first on signal when a first voltage signal that represents an inductor current of the first voltage conversion circuit is less than the feedback compensation signal, to activate a first power switch control signal to turn on a first power switch of the first voltage conversion circuit based on the first on signal, and to deactivate the first power switch control signal to turn off the first power switch after a predetermined time; and (iii) a second power switch control circuit configured to activate a second on signal after half of a switching period from a rising edge of the first on signal, and to activate a second power switch control signal to turn on a second power switch of the second voltage conversion circuit based on the second on signal, where the switching period comprises a time interval between a rising edges of the first on signal of previous and current switching periods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an example interleaved buck switching power supply.

FIG. 2 is a schematic block diagram of a first example control circuit of an interleaved switching power supply, in accordance with embodiments of the present invention.

FIG. 3 is a schematic block diagram of a second example control circuit of an interleaved switching power supply, in accordance with embodiments of the present invention.

FIG. 4 is a schematic block diagram of a third example control circuit of an interleaved switching power supply, in accordance with embodiments of the present invention.

FIG. 5 is a schematic block diagram of an example interleaved switching power supply control circuit with a current sharing function, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention may be described in conjunction with the preferred embodiments, it may be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it may be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, processes, components, structures, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention.

Main circuits of a switching power supply can be coupled in an “interleaved” fashion, and in parallel to decrease the burden of the power switches, to improve the power density of the power supply, and to decrease the ripple of the output voltage of the switching power supply in order to maintain a higher quality of output voltage. FIG. 1 shows an example of an interleaved switching power supply. In this example, the main circuit configuration can include two phase buck circuits coupled interleaved and in parallel, and the interleaved switching power supply can include a first voltage regulator, a second voltage regulator, and an output capacitor C_(o). The first voltage regulator can include main power switch (e.g., a power transistor) Q₁, inductor L₁, and free-wheeling diode D₁₁. The second voltage regulator can include main power switch (e.g., a power transistor) Q₂, inductor L₂, and free-wheeling diode D₁₂. Main power switches Q₁ and Q₂ can connect to the positive electrode of input supply V_(i), and to inductors L₁ and L₂, respectively.

Other terminals of inductor L₁ and inductor L₂ can connect to each other. For example, a first terminal of free-wheeling diode D₁₁ can connect to a second terminal of main power switch Q₁. A first terminal of free-wheeling diode D₁₂ can connect to a second terminal of main power switch Q₂. The second terminals of free-wheeling diodes D₁₁ and D₁₂ can both connect to the negative electrode of input supply V_(i). Output capacitor C_(o) can connect to the second terminals of inductor L₁ and inductor L₂, and to ground. The positive electrode and negative electrode of output capacitor C_(o) can be configured as the positive electrode and negative electrode of the output terminals of the interleaved switching power supply.

In operation of the interleaved switching power supply, the conduction phase of main power switch Q₂ may lag that of main power switch Q₁ by about 180°. As such, one of power switched Q₁ and Q₂ may be on, while the other is off, thus “interleaving” the operation of the power switches. Currents i_(L1) and i_(L2) can be generated, and ripple waves of current i_(L1) and i_(L2) can be counteracted to decrease the current ripple of output current i_(o). However, the frequency may be increased by, e.g., two times the original frequency, and the voltage ripple of output voltage V_(o) may also be decreased. Therefore, the values of inductor L₁, inductor L₂, and output capacitor C_(o) can be decreased to improve the dynamic response. For example, each buck circuit may only afford half of the output power, and current flowing through inductors L₁ and L₂, and power switches Q₁ and Q₂ may be only half of that of regular buck circuit. Also, in high frequency working conditions, ceramic capacitors of lower equivalent resistance can be utilized as output capacitor C_(o) to maintain the output of the switching power supply as steadier and with higher efficiency.

Phase difference control of the interleaved switching power supply can be achieved when a constant frequency control is employed. For typical switching power supplies, better dynamic response performance and simplified compensation design can be achieved by constant time control mode. However, when the constant on time control mode is used in interleaved switching power supplies, interleaved parallel control of the two voltage regulators may be relatively difficult due to the variable operation frequency. Power switches Q₁ and Q₂ may alternately turned on by 180°, and inductor currents i_(L1) and i_(L2) can be superimposed to decrease significantly the ripple of output current i_(o) and output voltage V_(o). Each voltage conversion circuit may provide half of the output power to decrease current stress of both power switches and inductors. Further, ceramic capacitors of lower equivalent resistance can be used in the output capacitor to improve dynamic performance, stability, and/or efficiency of the switching power supply in higher frequency operation.

Control of an interleaved switching power supply can include interleaved control for the power switches (e.g., Q₁ and Q₂) of the two voltage conversion circuits. Constant frequency control and constant on time control can be employed in some cases. However, dynamic performance may suffer for constant frequency control, and interleaved control may be difficult due to the variable frequency for constant on time control. In particular embodiments, a control circuit may be applied in an interleaved switching power supply, which can utilise an interleave control mode by 180° phase differences, and can improve dynamic performance, stability, and/or efficiency of the power supply.

In one embodiment, a control circuit configured for an interleaved switching power supply having first and second voltage conversion circuits, can include: (i) a feedback compensation signal generation circuit configured to calculate and compensate an output voltage sampled at output terminals of the interleaved switching power supply, and to generate a feedback compensation signal; (ii) a first power switch control circuit configured to activate a first on signal when a first voltage signal that represents an inductor current of the first voltage conversion circuit is less than the feedback compensation signal, to activate a first power switch control signal to turn on a first power switch of the first voltage conversion circuit based on the first on signal, and to deactivate the first power switch control signal to turn off the first power switch after a predetermined time; and (iii) a second power switch control circuit configured to activate a second on signal after half of a switching period from a rising edge of the first on signal, and to activate a second power switch control signal to turn on a second power switch of the second voltage conversion circuit based on the second on signal, where the switching period comprises a time interval between a rising edges of the first on signal of previous and current switching periods.

Referring now to FIG. 2, shown is a schematic block diagram of a first example control circuit of an interleaved switching power supply, in accordance with embodiments of the present invention. This example interleaved switching power supply can include a first voltage conversion circuit (e.g., including power switch Q₁ and inductor L₁) and a second voltage conversion circuit (e.g., including power switch Q₂ and inductor L₂) coupled in parallel. The control circuit can include feedback compensation signal generation circuit 100, and power switch control circuits 200 and 300.

Feedback compensation signal generation circuit 100 can generate feedback compensation signal V_(C1) by calculating and compensating output voltage V_(o) from the output terminals of the interleaved switching power supply. Power switch control circuit 200 can compare voltage signal V_(i,L1) that represents inductor current i_(L1) of the first voltage conversion circuit against feedback compensation signal W_(C1). When voltage signal V_(iL1) is less than feedback compensation signal W_(C1), on signal V_(S1) can be activated, and switch control signal V_(Q1) can be activated in accordance with on signal V_(S1). When on signal V_(S1) is active, switch control signal V_(Q1) is active to turn on power switch Q₁. After power switch Q₁ is turned on for predetermined time T_(on), switch control signal V_(Q1) can go inactive to turn off power switch Q₁.

Power switch control circuit 300 can receive on signal V_(S1) and may record moment or time point t₁ when the rising edge of the on signal V_(S1) of a previous switching period occurs, and moment or time point t₂ when the rising edge of on signal V_(S1) of the current switching period occurs. The time interval (t₂−t₁) of these two moments/time points can be utilized as switching period T of power switches Q₁ and Q₂. Another on signal (V_(S2)) can be generated after half of switching period (e.g., T/2) from a rising edge of on signal V_(S1). Power switch control signal V_(Q2) can then be generated in accordance with on signal V_(S2). When power switch control signal V_(Q2) is active, power switch Q₂ can be turned on for predetermined time T_(on), and then an off signal can be generated to make power switch control signal V_(Q2) inactive in order to turn off power switch Q₂.

In particular embodiments, a feedback compensation signal can be generated by calculating and compensation the output voltage, which is then be compared against voltage signal V_(iL1) to generate signal V_(S1). Power switch control signal V_(Q1) can then be generated in accordance with on signal V_(S1). When on signal V_(S1) is active, power switch control signal V_(Q1) can be active to turn on power switch Q₁. Also, a switching period of the switching power supply can be generated in accordance with time t₁ when the rising edge of on signal V_(S1) of a previous switching period occurs, and time/moment t₂ when a rising edge of on signal V_(S1) of the current switching period occurs (that is: T=t₂−t₁). On signal V_(S2) can be generated after half of the switch period T/2, and power switch control signal V_(Q2) can be generated in accordance with V_(S2). Power switch Q₂ can be turned on after half of switching period T/2 from the time of turning on main power switch Q₁. Therefore, power switches Q₁ and Q₂ can be turned on in an “interleaved” fashion. In this way, power supply control may not be influenced by the frequency of the power supply, which can improve the dynamic performance, stability, and/or efficiency of the power supply.

Referring now to FIG. 3, shown is a schematic block diagram of a second example control circuit of an interleaved switching power supply, in accordance with embodiments of the present invention. In this example, feedback compensation signal generation circuit 100 can include sampling circuit 110, transconductance operational amplifier Gm1, and compensation capacitor C₁. Sampling circuit 110 can sample output voltage V_(o) at the output terminals of the interleaved switching power supply, and may generate voltage sampling signal V_(S). For example, sampling circuit 110 can include two sampling resistors coupled in series, as shown. Transconductance operational amplifier Gm1 can calculate the error between reference voltage V_(ref1) at the non-inverting input terminal, and voltage sampling signal V_(S) at the inverting input terminal. Compensation capacitor C₁ can connect between the output terminal of transconductance operational amplifier Gm1 and ground, and may be charged by the output signal of transconductance operational amplifier Gm1. For example, a voltage across compensation capacitor C₁ may be employed as feedback compensation signal V_(C1), which can be provided to power switch control circuit 200.

Power switch control circuit 200 can include on signal generation circuit 210, off signal generation circuit 220, and flip-flop RS1. On signal generation circuit 210 can include comparator A₁, the non-inverting input terminal of which can receive feedback compensation signal V_(C1), the inverting input terminal of which can receive voltage signal V_(iL1), and the output terminal of which can output on signal V_(S1). Off signal generation circuit 220 can receive power switch control signal V_(Q1). When power switch control signal V_(Q1) is activated, a first timing can begin, and when the duration of the first timing reaches predetermined time T_(on), a first off signal may be generated. The set terminal of flip-flop RS1 can receive power switch control signal V_(Q1), the reset terminal can receive the first off signal, and the output terminal can output power switch control signal V_(Q1). When on signal V_(S1) is active, power switch control signal V_(Q1) may be active to turn on power switch Q₁. When the first off signal is active, power switch control signal V_(Q1) may be inactive to turn off power switch Q₁.

Power switch control circuit 300 can include on signal generation circuit 310, off signal generation circuit 320, and flip-flop RS2. On signal generation circuit 310 can receive on signal V_(S1). On signal V_(S2) may be generated after half of the switching period T/2 from the moment when the rising edge of on signal V_(S1) occurs. On signal generation circuit 320 can receive power switch control signal V_(Q2). When power switch control signal V_(Q2) is active, a second timing may begin, and when the duration of the second timing reaches predetermined time T_(on), a second off signal may be generated. The set terminal of flip-flop RS2 can receive on signal V_(S2), the reset terminal of flip-flop RS2 can receive the second off signal, and the output terminal can output power switch control signal V_(Q2). In this particular example, two flip-flops are employed to generate the control signals of corresponding power switch in accordance with corresponding on/off signals for interleaved operation of the two power switches. In this fashion, power supply control may not be influenced by the operating frequency, and which can improve the dynamic performance, stability, and/or efficiency of the power supply.

Referring now to FIG. 4, shown is a schematic block diagram of a third example control circuit of an interleaved switching power supply, in accordance with embodiments of the present invention. Off signal generation circuit 220 can include capacitor C₂, current source I₁, controllable switch S₁, and comparator A₂. Capacitor C₂, current source I₁, and controllable switch S₁ can be coupled in parallel, with one common node connecting to the non-inverting input terminal of comparator A₂, and another to ground. Both power switch Q₁ and controllable switch S₁ can be controlled by power switch control signal V_(Q1). When power switch control signal V_(Q1) is active, controllable switch S₁ may be turned off, and capacitor C₂ may be charged by current source I₁ to generate capacitor voltage V_(C2). When power switch control signal V_(Q1) is inactive, controllable switch S₁ may be turned on, and capacitor C₂ may be discharged through controllable switch S₁ until capacitor voltage V_(C2) is zero.

The non-inverting input terminal of comparator A₂ can receive capacitor voltage V_(C2), the inverting input terminal can receive reference voltage V_(ref2), and the output terminal can output the first off signal. For example, a value of capacitor C₂ and current source I₁ can be set such that the time that capacitor voltage V_(C2) takes to reach a level of reference voltage V_(ref2) when capacitor C₂ is charged by current source I₁ may be consistent (e.g., the same, or within a predetermined amount) with predetermined time T_(on). The first timing may be determined by the charging to capacitor C₂. Also, on signal generation circuit 310 can include buffer B₁, buffer B₂, buffer B₃, flip-flop RS3, flip-flop RS4, flip-flop RS5, voltage generation circuit 311, voltage generation circuit 312, and comparator A₃. Buffer B₁ can receive on signal V_(S1), and may generate a first buffer signal, buffer B₂ can receive the first buffer signal to generate a second buffer signal, and buffer B₃ can receive on signal V_(S1), and may generate a third buffer signal.

The set terminal of flip-flop RS3 can receive the first buffer signal, the reset terminal can receive the second buffer signal, and the output terminal can output a second controllable signal. The set terminal of flip-flop RS4 can receive the first on signal, the reset terminal can receive the third buffer signal, and the output terminal can output a third controllable signal. The set terminal of flip-flop RS5 can receive the on signal V_(S1), the reset terminal can receive the on signal V_(S2), and the output terminal can output a fourth control signal. Voltage generation circuit 310 can receive the second and third control signals, and may generate a fourth voltage. Voltage generation circuit 320 can receive the fourth control signal, and may generate a fifth voltage. The inverting input terminal of comparator A₃ can receive the fourth voltage, the non-inverting input terminal can receive the fifth voltage, and the output terminal can output on signal V_(S2).

For example, voltage generation circuit 310 can include controllable switch S₂, controllable switch S₃, capacitor C₄, and current source I₂. Controllable switch S₂, controllable switch S₃, and capacitor C₄ can be coupled in parallel, where one common node of which can connect to a power terminal of controllable switch S₃, and the other common node of which can connect to ground. One terminal of capacitor C₄ can connect to the other power terminal of controllable switch S₃, and may be coupled to the inverting input terminal of comparator A₃, the other terminal of capacitor C₄ can connect to ground. Also, a voltage across capacitor C₄ may be configured as the fourth voltage.

Controllable switch S₂ may be controlled by the second control signal. When the second control signal is inactive, controllable switch S₂ may be off, and capacitor C₃ may be charged by current source I₂. When the second control signal is active, controllable switch S₂ may be on, and capacitor C₃ may be discharged. When controllable switch S₂ is turned on, capacitor voltage V_(C3) across capacitor C₃ may have maximum value. Controllable switch S₃ may be controlled by the third control signal generated by flip-flop RS4. When this third control signal is active, controllable switch S₃ may be on to maintain voltage V_(C4) as consistent with capacitor voltage V_(C3). When the third control signal is inactive, controllable switch S₃ may be off, and voltage V_(C4) can maintain the maximum value of capacitor voltage V_(C3).

Voltage generation circuit 312 can include controllable switch S₄, capacitor C₅, and current source I₃ coupled in parallel, where one common node of which can connect to non-inverting input terminal of comparator A₃, and the other common node of which can connect to ground. Controllable switch S₄ may be controlled by the fourth control signal generated by flip-flop RS5. When the fourth control signal is inactive, controllable switch S₄ may be turned off, and capacitor C₅ may be charged by current source I₃. For example, a voltage across capacitor C₅ may be configured as the voltage V_(C5). When voltage V_(C5) reaches the maximum value of the capacitor voltage V_(C3), the output signal of comparator A₃ may go high to generate on signal V_(S2).

For example, the values of capacitor C₃ and current source I₂ may be set correspondingly to guarantee that the time taken for voltage V_(C3) to be generated by charging capacitor C₃ via current source I₂ to reach its maximum value is consistent with switching period T of power switch Q₁. For voltage generation circuits 311 and 312, the value of capacitor C₃ may be set as twice that of the value of capacitor C₅, or the value of current source I₃ may be set as twice of the value of current source I₂. Therefore, the charging time of capacitor C₃ may be twice that of the charging time of capacitor C₅, and the charging time of capacitor C₅ may be half that of the switching period T/2. The time interval of the moment when the rising edge of on signal V_(S1) occurs and the moment when the rising edge of on signal V_(S2) occurs may be half that of the switching period (e.g., T/2), in order to achieve interleaved operation of main power switches Q₁ and Q₂ of the switching power supply.

Off signal generation circuit 320 can include capacitor C₆, current source I₄, controllable switch S₅, and comparator A₄. For example, capacitor C₆, current source I₄, controllable switch S₅, and comparator A₄ can be coupled in parallel, with one common node connecting to a non-inverting input terminal of comparator A₄, and another common node connecting to ground. Controllable switch S₅ may be controlled by a control signal (e.g., V_(Q2)) of the power switch Q₂. When control signal V_(Q2) is active, controllable switch S₅ may be off, and capacitor C₆ may be charged by current source I₄ to generate capacitor voltage V_(C6).

When control signal V_(Q2) is inactive, controllable switch S₅ may be on, and capacitor C₆ may be discharged until capacitor voltage V_(C6) decreases to zero. Comparator A₄ can receive capacitor voltage V_(C6) at its non-inverting input terminal, and reference voltage V_(ref3), and may generate the second off signal at its output terminal. For off signal generation circuit 320, capacitor C₆ may be charged by current source I₄, and values of capacitor C₆ and current source I₄ can be determined such that the time taken for capacitor voltage V_(C6) to increase to a level of reference voltage V_(ref3) from zero is consistent with predetermined time T_(on). Therefore, the second timing may be achieved by the charging of capacitor C₆.

Referring now to FIG. 5, shown is a schematic block diagram of an example interleaved switching power supply control circuit with a current sharing function, in accordance with embodiments of the present invention. This example control circuit can include feedback compensation signal generation circuit 100, power switch control circuit 200, and power switch control circuit 300. Power switch control circuit 200 can include on signal generation circuit 210, off signal generation circuit 220, and RS flip-flop RS1. Power switch control circuit 300 can include on signal generation circuit 310, off signal generation circuit 320, current sharing circuit 330, and RS flip-flop RS2.

The structure and working principles of feedback compensation signal generation circuit 100, power switch control circuit 200, and on signal generation circuit 310, off signal generation circuit 320, and RS flip-flop RS2 of power switch control circuit 300 have been described above. In the particular example of FIG. 5, current sharing circuit 330 can receive voltage signals V_(iL1) and V_(iL2), and may generate current signal i₆. When controllable switch S₅ is off, capacitor C₆ may be charged by both current signal i₆ and current source I₄ to maintain inductor current i_(L1) as consistent with inductor current i_(L2).

For example, current sharing circuit 330 can include transconductance operational amplifier Gm2, transconductance operational amplifier Gm3, and capacitor C₇. Transconductance operational amplifier Gm2 can receive voltage signal V_(iL2) at its non-inverting input terminal, and voltage signal V_(iL1) at its inverting input terminal, and may generate current signal i₅ that represents an error between voltage signals V_(iL1) and V_(iL2). One terminal of capacitor C₇ can connect to the output terminal of transconductance operational amplifier Gm2, and the other terminal can connect to ground. Capacitor C₇ may be charged by current signal i₅ to generate capacitor voltage V_(C7).

The non-inverting input terminal of transconductance operational amplifier Gm3 can receive capacitor voltage V_(C7), and the inverting input terminal can connect to ground. The output terminal of transconductance operational amplifier Gm3 can generate current signal i₆. Inductor currents i_(L1) and i_(L2) can be compared by transconductance operational amplifier Gm2 to generate current signal i₅ that represents an error between inductor currents i_(L1) and I_(L2), and which may be employed to charge compensation capacitor C₇ to generate capacitor voltage V_(C7). Transconductance operational amplifier Gm3 can compare capacitor voltage V_(C7) and zero.

When inductor current i_(L1) is consistent with inductor current i_(L2), the output signal of transconductance operational amplifier Gm3 may be zero. When inductor current i_(L1) is inconsistent with inductor current i_(L2), current signal i₆ may be generated by transconductance operational amplifier Gm3, which may be provided to off signal generation circuit 320. Capacitor C₆ may be charged by the current signal i₆ and current source I₄. The charging time of capacitor C₆ may be controlled to regulate the generation time of the second off signal, as input to the reset terminal of flip-flop RS2. In this way, the on time of power switch Q2 may be regulated such that inductor current i_(L1) is consistent with inductor current i_(L2).

The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. 

What is claimed is:
 1. A control circuit configured for an interleaved switching power supply having first and second voltage conversion circuits, the control circuit comprising: a) a feedback compensation signal generation circuit configured to calculate and compensate an output voltage sampled at output terminals of said interleaved switching power supply, and to generate a feedback compensation signal; b) a first power switch control circuit configured to activate a first on signal when a first voltage signal that represents an inductor current of said first voltage conversion circuit is less than said feedback compensation signal, to activate a first power switch control signal to turn on a first power switch of said first voltage conversion circuit based on said first on signal, and to deactivate said first power switch control signal to turn off said first power switch after a predetermined time; and c) a second power switch control circuit configured to activate a second on signal after half of a switching period from a rising edge of said first on signal, and to activate a second power switch control signal to turn on a second power switch of said second voltage conversion circuit based on said second on signal, wherein said switching period comprises a time interval between a rising edges of said first on signal of previous and current switching periods.
 2. The control circuit of claim 1, wherein said feedback compensation signal generation circuit comprises: a) a sampling circuit configured to sample said output voltage, and to generate a voltage sampling signal; b) a first transconductance operational amplifier configured to determine an error between a first reference voltage and said voltage sampling signal; and c) a first compensation capacitor coupled between an output of said first transconductance operational amplifier and ground, wherein a voltage across said first compensation capacitor is configured as said feedback compensation signal.
 3. The control circuit of claim 1, wherein said first power switch control circuit comprises: a) a first comparator configured to receive said feedback compensation signal and said first voltage signal, and to generate said first on signal; b) a first off signal generation circuit configured to activate a first off signal said predetermined time after said first power switch control signal is activated; and c) a first flip-flop configured to generate said first power switch control signal in response to said first on signal and said first off signal.
 4. The control circuit of claim 3, wherein said first off signal generation circuit comprises: a) a second capacitor, a first current source, and a first controllable switch coupled in parallel, and having a common node coupled to ground; b) said first controllable switch being off and said second capacitor being charged by said first current source to generate a second capacitor voltage when said first power switch control signal is active, said first controllable switch being on and second capacitor voltage being discharged to zero when said first power switch control signal is inactive; and c) a second comparator configured to activate said first off signal in response to said second capacitor voltage and a second reference voltage, wherein a value of said second capacitor and said first current source are determined such that a time duration for said second capacitor voltage to reach a level of said second reference voltage from zero is consistent with said predetermined time.
 5. The control circuit of claim 1, wherein said second power switch control circuit comprises: a) a second on signal generation circuit configured to generate said second on signal after said half of said switching period from said rising edge of said first on time signal; b) a second off signal generation circuit configured to activate a second off signal said predetermined time after said second power switch control signal is activated; and c) a second flip-flop configured to generate said second power switch control signal based on said second on signal and said second off signal.
 6. The control circuit of claim 5, wherein said second on signal generation circuit comprises: a) a first buffer configured to receive said first on signal, and to generate a first buffer signal; b) a second buffer configured to receive said first buffer signal, and to generate a second buffer signal; c) a third buffer configured to receive said first on signal, and to generate a third buffer signal; d) a third flip-flop configured to receive said first buffer signal and said second buffer signal, and to generate a second control signal; e) a fourth flip-flop configured to receive said first on signal and said third buffer signal, and to generate a third control signal; f) a fifth flip-flop configured to receive said first on signal and said second on signal, and to generate a fourth control signal; g) a fourth voltage generation circuit configured to receive said second and third control signals, and to generate a fourth voltage; h) a fifth voltage generation circuit configured to receive said fourth control signal, and to generate a fifth voltage; and i) a third comparator configured to receive said fourth voltage and said fifth voltage, and to generate said second on signal.
 7. The control circuit of claim 6, wherein said fourth voltage generation circuit comprises: a) a second controllable switch, a third capacitor, and a second current source coupled in parallel, and having a first common node coupled to a third controllable switch, and a second common node coupled to ground; b) a fourth capacitor coupled to said third controllable switch and said third comparator, wherein a voltage across said fourth capacitor is configured as said fourth voltage; c) said second controllable switch being off and said third capacitor being charged by said second current source when said second control signal is active, and being on when said second control signal is inactive, wherein a voltage across said third capacitor reaches a maximum value when said second controllable switch is on, and wherein a value of said third capacitor and said second current source are determined such that a time duration for said voltage across said third capacitor to reach said maximum value is consistent with said switching period; and d) said third controllable switch being on to maintain said fourth voltage consistent with said voltage across said third capacitor when said third control signal is active, and said third controllable switch being off to maintain said fourth voltage as consistent with said maximum value when said third control signal is inactive.
 8. The control circuit of claim 7, wherein said fifth voltage generation circuit comprises: a) a fourth controllable switch, a fifth capacitor, and a third current source coupled in parallel and having a first common node coupled to said third comparator, and a second common node coupled to ground; and b) said fourth controllable switch being off and said fifth capacitor being charged by said third current source when said fourth control signal is inactive, wherein a voltage across said fifth capacitor is configured as said fifth voltage, and a capacitance of said third capacitor is twice of that of said fifth capacitor.
 9. The control circuit of claim 5, wherein said second off signal generation circuit comprises: a) a sixth capacitor, a fourth current source, and a fifth controllable switch coupled in parallel with a common node coupled to ground; b) said fifth controllable switch being off and said sixth capacitor being charged by said fourth current source to generate a sixth capacitor voltage when said second power switch control signal is active, and said fifth controllable switch being on and said sixth capacitor being discharged to zero when said second power switch control signal is inactive; and c) a fourth comparator configured to receive said sixth capacitor voltage and a third reference voltage, and to generate said second off signal, wherein a value of said sixth capacitor and said fourth current source are determined such that a duration for said sixth capacitor voltage to reach a level of said third reference voltage from zero is consistent with said predetermined time.
 10. The control circuit of claim 9, wherein said second power switch control circuit comprises a current sharing circuit configured to generate a sixth current signal in accordance with said first and second voltage signals, and wherein said sixth current signal is configured to charge said sixth capacitor when said fifth controllable switch is off.
 11. The control circuit of claim 10, wherein said current sharing circuit comprises: a) a second transconductance operational amplifier configured to generate a fifth current signal based on said first and second voltage signals; b) a seventh capacitor coupled to said second transconductance operational amplifier and ground, wherein said seventh capacitor is configured to be charged by said fifth current signal to generate a seventh capacitor voltage; and c) a third transconductance operational amplifier configured to generate said sixth current source based on said seventh capacitor voltage. 